Use the mouse to aim and shoot.

Alien Assault is a Fast Paced Defense Game. Hundreds of alien creatures are trying to blow up your base.
Luckily you have soldiers as well as your own weapon to help you defend the building. At the start of each wave of attacks you
will be shown where the invasion is coming from, so use those few seconds to rearrange your troops and give them orders,
you can do this by holding the left mouse key down over the character. Watch your ammo level, once it’s gone, you will have to look for pickups.